The present invention relates to low molecular weight fully fluorinated hexafluoropropylene oxide (HFPO) oligomers of the formula: ##STR2## where n is 1 to 9, for use as heat transfer media in vapor phase heat transfer processes which use contact with heated vapor to raise the temperature of a workpiece.
Generally, vapor phase heat transfer or direct contact condensation heating involves boiling a liquid to generate a body of hot saturated vapor in equilibrium with the boiling liquid. The liquid should have a boiling point at least equal to the temperature at which a soldering, fusing, brazing, curing or other similar heat transfer operation is to be performed. The workpiece to be heated is immersed in the hot saturated vapor generated by the pool of boiling liquid. The vapor, at the temperature of the boiling liquid, envelops the workpiece and begins to condense, giving up its latent heat of vaporization. The workpiece is thereby heated rapidly and uniformly to the temperature of the boiling liquid and the soldering, fusing, brazing, curing or other heat transfer process occurs.
Vapor phase soldering is a special case of the general process of vapor phase heat transfer. In a reflow soldering process, heat melts solder which has been previously applied to the desired locations on a workpiece. Although the present invention is not to be construed as limited to soldering, its background is most readily understood in the context of soldering.
In a vapor phase heat transfer operation and, in particular, a vapor phase soldering process, the heat transfer medium should have certain characteristics. It is important that the medium have the correct boiling point so as to melt the solder without damaging the workpiece. It should also have high thermal and chemical stability so as to withstand long periods of boiling while in contact with many different metals and organic compounds, be compatible with the work assembly itself, have low heats of vaporization so as to evaporate rapidly from the surface of the work assembly after the soldering is complete, and not leave undesirable residues after evaporation from the work assembly. Generally, heat transfer liquids should produce vapors that are heavier than air so as to facilitate their confinement and thereby help control losses during operation. Non-flammablability of the heat transfer medium is particularly desirable to avoid fires. Low flux solubility is important to avoid excessive flux removal during heat-up of the work assembly. Work pieces usually enter and exit an apparatus designed for vapor phase soldering through openings through which some loss may occur to the surrounding work area of both the heat transfer medium and by-products which may be formed during the prolonged boiling of the liquid; therefore, low toxicity of both the liquid and any by-products is required. In addition, properties of the heat transfer medium should make it possible to minimize these losses for reasons of economy as well as safety.
Heat transfer media currently used in vapor phase heat transfer processes have the afore described characteristics to varying degrees. The following references describe various heat transfer media and processes.
U.S. Pat. No. 3,866,307 (1975) to Pfahl discloses that fluorocarbons such as fluorinated polyoxypropylene can be used as heat transfer liquids for vapor phase heat transfer operations. Two specific trademarked products designated "Freon" E5 and "Freon" E4 which were sold by E.I. duPont de Nemours & Co. are described.
U.S. Pat. No. 4,032,033 (1977) to Chu discloses an electrochemically fluorinated amine product sold by 3M Company under the tradename "Fluorinert FC-70" for use as a heat transfer liquid.
U.S. Pat. No. 4,032,033 also discloses the use of multiple heat transfer media in vapor phase heat transfer applications. Secondary media interposed between a body of hot primary vapor and the atmosphere can reduce the losses of primary vapor into the surrounding work area. The secondary medium must have a vapor less dense than the saturated vapor formed from the primary heat transfer liquid but more dense than air at working temperatures and pressures.
U.S. Pat. No. 4,549,686 (1985) to Sargent claims a method of soldering in which the vapor bath is composed predominantly of perfluorotetradecahydrophenanthrene (C.sub.14 F.sub.24), tradename "Flutec PP11".
While the heat transfer media described in the foregoing references are useful in vapor phase heat transfer processes, it is desirable to use a heat transfer medium with improved characteristics, especially one which has lower flux solubility and one which forms smaller quantities of toxic by-products during prolonged use.